a. Field of the Invention
The present invention pertains generally to wind turbines and more particularly to cross-flow wind turbines.
b. Description of the Background
Wind turbines have provided a valuable source of alternative energy. Constant improvements in wind turbine systems have been made over the past few years. Most of the wind turbine systems in use today are axial-flow systems that are otherwise known as propeller systems. In axial-flow wind turbine systems, air travels in a direction that is substantially parallel to the rotating axis of the wind turbine. The large propeller blades are driven by the wind at a rotational speed that creates tip speeds of the blade that can range from approximately six to nine times the speed of the wind. Tip speeds of over several hundred miles an hour can be achieved in moderately strong to strong winds which will cause axial-flow wind turbines to break apart. Hence, expensive braking systems and systems for feathering the angle of the blades must be used to control the speed of axial-flow wind turbines in moderately strong to strong winds. As a result, axial-flow wind turbines are inefficient in moderately strong to strong winds and, as such, are unable to extract power from wind energy when it is at its greatest energy producing level. In effect, axial-flow wind turbines are designed for use in light to moderate winds.
Further, axial-flow wind turbines are very dangerous to birds. Birds are frequently killed by the inability of these avians to sense the presence of the rapidly moving blades of the axial-flow wind turbines.
Cross-flow wind turbines constitute a different class of wind turbines. See U.S. patent application Ser. No. 10/831,515, entitled “Wind Turbine Having Airfoils for Blocking and Directing Wind and Rotors With or Without a Central Gap,” filed Apr. 23, 2004, and U.S. Pat. No 6,015,258, entitled “Wind Turbine,” issued Jan. 18, 2000, the entire contents of which is hereby incorporated herein by reference for all it discloses and teaches. In cross-flow wind turbines, the wind flows across the rotors in a direction that is substantially normal to the axis of the rotating shaft. Cross-flow wind turbines have certain advantages over axial-flow wind turbines, but have not previously been seriously considered as a source of alternative energy because of the comparatively lower efficiencies with respect to the axial-flow wind turbines. An advantage of the cross-flow wind turbine is that the tip speed of the rotors of the cross-flow wind turbines move at a maximum speed that is only slightly greater than the speed of the wind. As such, cross-flow turbines can operate equally as well in both low and high wind conditions. In addition, the cross-flow wind turbine is easily seen by birds resulting in no avian deaths. Cross-flow wind turbines are also much quieter than the axial-flow wind turbines, primarily because the cross-flow wind turbines do not move at a high rate of speed and do not create a substantial amount of turbulence.